The grand machinery of galaxies is set in motion by some of the most basic forces in the
universe. The gravitational energy between atoms and molecules in interstellar gas is continually
converted into light, heat, and high-speed particles in the cores of stars. The by-products of
this ongoing process become the essential ingredients of planets and life itself. In addition,
the formation of stars and the release of matter and energy during their lifetimes regulates the
inevitable crush of gravity, setting up a complex feedback loop that allows galaxies to continue
this cycle for billions of years.

A 50°-long ionized filament rising out of the Galactic plane, as observed in Hα by
WHAM.

The Wisconsin H-Alpha Mapper (WHAM) group is studying one important component of the
interstellar medium (ISM) in our own Milky Way to help answer important questions about how
galaxies work. Where does the energy produced in the star-forming regions of our Galaxy go? How
does that energy propagate away from these birth sites? How does this energy change as it
travels, and how is it deposited back into the Galaxy?

The disk of the Milky Way contains a thick (many thousands of light-years) layer of ionized
gas, dubbed the Warm Ionized Medium (WIM). This layer appears to be powered by ongoing, active
star formation. Ultraviolet and X-ray light leaking from the dense star-forming regions in the
disk appears to be the primary source of energy. But some evidence suggests that a portion of the
power may come from converting energy released in supernovae, which occur on average about once a
century in the Milky Way. Due to the WIM's diffuse nature—only about 100,000 atoms per
cubic meter—it is difficult to detect and characterize with traditional astronomical
instruments.

WHAM is a custom-built observatory designed for studying the WIM in detail. It has produced the
first map that traces not only the distribution but also the motion of the gas. To achieve this
goal, WHAM obtains spectra instead of images of very faint Balmer-alpha (Hα)
emission from ionized hydrogen. Its primary mission is to produce the first spectral, all-sky
survey of this emission from the Milky Way. While on
Kitt Peak in Arizona, WHAM obtained the data for the
northern portion of this survey and followed up with a variety of other projects to explore the
properties of the WIM and the origin of the energy needed to sustain it. In 2009, we moved WHAM
to
Cerro Tololo in Chile so that it can observe from the
southern hemisphere and complete the all-sky survey. We will then return to exploring the
detailed physics of the WIM as well as gas associated with two of the Milky Way's satellite
galaxies, the Large and Small Magellanic Clouds. Most of these ongoing projects combine new WHAM
emission-line observations of elements other than hydrogen (primarily sulfur, nitrogen, oxygen,
and helium) to measure the physical conditions of the gas and explore the processes involved in
powering the WIM.